Medically Reviewed By: Dr Paul Lim – MBBS, M.Med (Int Med), MRCP (UK), FAMS (Cardiology)
Heart block is a delay in the heart’s electrical wiring, not a blocked artery, and its mildest grade is also its commonest one. Just seen it on an ECG report? Bring the report to Dr Paul Lim, who is an easy person to ask, and you will leave knowing which grade you have and what it changes.
Dr Paul Lim Chun Yih
Senior Consultant Cardiologist & Electrophysiologist
Senior Consultant Cardiologist & Cardiac Electrophysiologist
Dr Paul Lim subspecialises in the heart’s electrical system, which is exactly what heart block affects. He grades conduction disease on the ECG and on ambulatory monitoring, looks for the reversible causes first, and implants pacemakers where one is needed. He completed advanced fellowship training at Barts Heart Centre, London under Singapore’s HMDP award.
Every heartbeat starts as an electrical signal in the sinus node, the heart’s own pacemaker in the top right chamber. The signal spreads across the upper chambers, funnels through a relay station called the atrioventricular (AV) node, passes into a short trunk called the bundle of His, and then races down its two branches to the lower chambers, which do the actual pumping. Heart block is what happens when that signal is slowed or stopped along the way.
Where the hold-up occurs gives the condition its two names. A delay at the AV node or just below it is atrioventricular block, graded first, second or third degree by how much of the signal survives the journey. A delay in one of the two branches further downstream is bundle branch block, which is a different finding with a different meaning, covered further down this page.
Heart block is an electrical problem: the wiring conducts poorly. A blocked artery is a plumbing problem, in which the vessels supplying the heart muscle narrow with fatty deposits. That is coronary artery disease, and it has different symptoms, different tests and different treatments. You can have either without the other, though long-standing artery disease is one of the things that damages the wiring over time.
All of it comes down to one relationship: does each small P wave (the upper chambers firing) reach the tall QRS complex (the lower chambers responding), and how long does it take?
Heart block at the AV node is graded by how much of the signal reaches the lower chambers, and reports write it as 1st degree AV block, 2nd degree AV block or 3rd degree AV block. The grade largely decides whether a pacemaker is needed.
| Type | What happens | Typical symptoms | Pacemaker needed? |
|---|---|---|---|
| First-degree | Every signal gets through, just more slowly | Usually none | No |
| Second-degree, Type I (Wenckebach) | Signal delay builds until one beat is dropped | Often mild or none | Not usually |
| Second-degree, Type II (Mobitz II) | Beats are dropped suddenly, without warning | Dizziness, fainting | Often yes |
| Third-degree (complete) | No signals get through; chambers beat independently | Severe; can be life-threatening | Yes |
The relay is slow but reliable. Every signal from the upper chambers reaches the lower chambers, taking longer than the standard fifth of a second to do it, and the beat that results is normal. Most people have no symptoms and would never have known without an ECG, so 1st degree heart block is almost always an incidental finding. It turns up in fit young adults, where high vagal tone slows the AV node during rest and sleep, and it becomes steadily more common with age.
What matters is what sits behind it, since medication is a frequent culprit, and whether it stays put. A first-degree block unchanged on your tracings for a decade is a different proposition from one that has appeared this year alongside new symptoms.
Some beats now fail to make it. Second-degree AV block comes in two types, and telling them apart changes the treatment.
Nothing gets through, and a report may shorten this to CHB. The upper chambers carry on at their usual rate, the lower chambers stop receiving instructions, and after a pause they fall back on a built-in escape rhythm of their own. How fast that is depends on where in the system it starts. An escape arising just below the AV node produces a narrow beat at around 40 to 60 a minute, and is comparatively steady. One arising lower down produces a wide beat, commonly below 40 and sometimes a good deal slower, and is less dependable. Those figures are approximate, and where the escape starts matters more than how fast it runs. Either way it is a safety net rather than a solution: it can be unreliable, and it may stop.
People in complete heart block often feel profoundly tired and breathless, and many faint. Sudden blackouts caused this way have their own name, Stokes-Adams attacks, and they happen without any warning at all. Complete heart block is treated urgently, with temporary pacing if needed while the cause is sorted out, followed by a permanent pacemaker in most cases.
Not sure which grade your report means? Bring it to Dr Paul Lim and have it explained to you.
Below the AV node the bundle of His splits into two branches, one for each lower chamber. If a branch stops conducting, the chamber it serves is activated late instead: the signal reaches it from the other side, spreading cell by cell, which takes longer and widens the QRS complex on the ECG. That is how both are diagnosed. The beat still arrives, which is the important difference from AV block.
| Right bundle branch block (RBBB) | Left bundle branch block (LBBB) | |
|---|---|---|
| How common | Common. Around 11 per cent of people by age 80 | Uncommon overall, roughly 0.06 to 0.1 per cent, but 6 to 7 per cent by age 80 |
| On the ECG | Wide QRS with an RSR′ pattern in the right-sided chest leads | QRS wider than 120 milliseconds, broad and notched in the left-sided leads |
| What it usually means | Often nothing. Benign when it stands alone | Often an underlying heart condition |
| What follows | A baseline tracing on file, and no treatment | An echocardiogram, and an assessment of the heart arteries |
| Pacemaker? | No | Only where heart failure calls for resynchronisation |
RBBB becomes more common with every decade, reaching around 11 per cent of people by the age of 80. A first appearance still deserves a proper look, to confirm there is nothing underneath it and to put a baseline tracing on file. That baseline is worth a great deal if you ever turn up somewhere with chest pain, because a doctor who cannot tell an old RBBB from a new one is working blind. You may also see incomplete RBBB reported, which is the same pattern in milder form and carries even less weight.
LBBB is far rarer, though it climbs steeply with age: 1 to 5 per cent past 70, and 6 to 7 per cent past 80. It also carries more weight. Where RBBB is often an isolated quirk of the wiring, LBBB keeps company with conditions that matter: coronary artery disease, a weakened or enlarged heart muscle, and disease of the aortic valve. Roughly a third of people with heart failure have it, which is why a newly discovered LBBB earns a search for a cause rather than reassurance. It also masks much of what the rest of the tracing would otherwise show. A milder incomplete LBBB is reported the same way. Where it sits alongside heart failure, a specialised pacemaker can bring the two sides back into step: see resynchronisation below.
The left branch itself divides into smaller fascicles, so a report may name a left anterior fascicular block (LAFB) or, less often, a left posterior one. These are minor findings on their own. Bifascicular block describes two of these pathways conducting poorly at once, most often RBBB together with left anterior fascicular block. It is worth knowing about because fewer intact routes remain, so it is followed more attentively, particularly if fainting enters the picture.
Mild grades tend to announce nothing. Symptoms appear once dropped beats start to leave the body short of the blood flow it needs.
Some causes are permanent changes to the conduction tissue. Others are entirely reversible, which is why they are looked for first.
Age is the commonest reason by some distance. The specialised fibres that carry the signal gradually give way to fibrous tissue, and conduction slows. This accounts for a large share of the pacemakers implanted for heart block, and it explains why the condition is far more frequent past 65.
Several widely used heart medicines slow AV conduction deliberately, and occasionally they slow it too much. Beta-blockers, certain calcium-channel blockers such as verapamil and diltiazem, digoxin and some antiarrhythmic drugs are the usual candidates, particularly in combination or when kidney function changes. Never stop a heart medicine on your own account; the review belongs with the doctor who prescribed it.
A previous heart attack can destroy conduction tissue outright, and the arteries that supply the AV node are themselves vulnerable to narrowing. Cardiac surgery and valve procedures can bruise or interrupt the pathways, so heart block is a recognised complication of both. Inflammation of the heart muscle, whether from a viral myocarditis or an infection seeded on a valve, is another route.
A raised potassium level is the clearest of these, and conduction often recovers once it is corrected. An underactive thyroid slows the heart generally, but it is an uncommon cause of block and treating it does not reliably reverse one. High vagal tone is the normal night-time slowing seen in trained athletes and many healthy young adults, and it can produce first-degree or Wenckebach block that disappears on waking or on exercise. Obstructive sleep apnoea drives that vagal effect harder.
Infiltrative conditions such as cardiac sarcoidosis and amyloidosis can involve the conduction system, and are worth considering when block appears in someone younger than expected. A small number of people are born with complete heart block. Lyme disease is a recognised cause where the tick that carries it is established, though it is not something acquired in Singapore.
Most reports naming a conduction abnormality are not urgent. A few are.
Do not wait for an appointment if you have:
Arrange a review soon if:
Worth clarifying, but rarely alarming:
Not sure which column you fall into? Speak to a cardiologist rather than guess from the report.
Have a concern about a heart block? Talk to our friendly Dr Paul Lim
The grade has to be caught on a tracing, and higher-grade block is often intermittent, so the work is partly a matter of recording at the right moment.
What the symptoms are, when they happen, and whether they tie to exertion or to rest. Every medicine is gone through, including the ones prescribed elsewhere, since rate-slowing drugs are one of the more common and most fixable contributors.
A few painless minutes, and the single most informative test here. It shows the degree of AV block, whether a bundle branch is involved, and how wide the QRS complexes have become. If you have an older tracing, bring it: the comparison is often more useful than either recording alone. Terms printed on a report are decoded on our abnormal ECG guide.
A resting ECG catches only the moment it is running. Where symptoms come and go, the recording has to go home with you:
An ultrasound scan of the heart, checking the pumping strength, the chamber sizes and the valves. Particularly important where a left bundle branch block has been found, because that is where an underlying condition is most likely to be hiding.
Thyroid function, potassium and the other electrolytes, and kidney function, since the last of these governs how much of a rate-slowing drug is actually circulating.
How the block behaves under exertion is informative in its own right. Block that improves as the heart rate climbs usually sits at the AV node and is the more benign kind; block that worsens with exercise points lower down the system and is treated more assertively.
Where the picture stays unclear and the stakes are high, fine wires passed to the heart can measure conduction directly and pin down exactly where the hold-up is. This is reserved for cases the non-invasive tests have left unresolved.
The ECG, Holter and echocardiogram are all done in clinic. Book a review with Dr Paul Lim.
Treatment follows the grade and the symptoms. Anything reversible is dealt with before a device is discussed.
First-degree block without symptoms, Wenckebach without symptoms, and an isolated bundle branch block in an otherwise healthy heart are all managed by observation. That means a baseline record, a plan for what would prompt another look, and a repeat ECG at sensible intervals.
Where a medicine is responsible, adjusting or replacing it may restore normal conduction entirely, and that decision is made together with whoever prescribed it. The same applies to a raised potassium level, and to untreated sleep apnoea. An underactive thyroid is worth treating in its own right, though correcting it reverses the block less often than you might expect. This step comes first because a pacemaker fitted for a problem that a tablet change would have fixed is a pacemaker you keep for life.
Where conduction has failed permanently, no drug restores it. A small device sits under the skin below the collarbone, watches every beat, and steps in only when a signal fails to arrive. It is recommended for symptomatic heart block of any grade, and for Mobitz II and complete heart block whether or not symptoms have appeared yet. Implantation is a routine procedure done under local anaesthetic with sedation, usually with an overnight stay.
Learn about pacemaker implantation →If complete heart block causes collapse, or the escape rhythm is dangerously slow, a temporary pacing wire can hold the rate up in hospital while the cause is established. Some people need nothing further, because the block was driven by something that then resolves. Most go on to a permanent device.
Left bundle branch block with heart failure is a particular case. Because the left side contracts late, the heart works against itself, and a device that paces both lower chambers can restore the timing. Cardiac resynchronisation therapy is considered where the echocardiogram shows reduced pumping strength and the QRS is broad, and the case for it is strongest once the QRS reaches about 150 milliseconds. A narrow QRS gets no benefit from it. It treats the heart failure as much as the conduction problem.
Most people with heart block, including everyone who ends up with a pacemaker, get on with life normally.
See Dr Paul Lim about a conduction abnormality on your ECG — which grade it is, and what it means for you.
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The questions people ask most often after a report comes back naming a conduction abnormality.
Heart block is a delay or failure of the electrical signal that travels from the heart’s upper chambers to its lower chambers. It is graded by how much of the signal gets through: first-degree, where every signal conducts but slowly; second-degree, where some beats are dropped; and third-degree or complete block, where none conduct at all. The grade largely determines whether treatment is needed.
No. Heart block is a fault in the heart’s electrical wiring, so a signal is delayed or fails to pass through. A blocked artery is a narrowing of the vessels that supply blood to the heart muscle, which is coronary artery disease. The two have different symptoms, different tests and different treatments. The similar wording is a common source of confusion, and worth clarifying at your consultation.
First-degree heart block usually causes no symptoms and often needs no treatment. Every electrical signal still reaches the lower chambers; it simply takes longer to get there. It is found in healthy people, including trained athletes, and becomes more common with age. Your cardiologist will still want to know what is behind it, review any medication that may be slowing conduction, and check whether it stays stable over time.
Both are second-degree heart block, and the difference matters a great deal. In Mobitz I, also called Wenckebach, the delay lengthens beat by beat until one beat is dropped, and then the pattern resets. It is usually the milder of the two and often needs no treatment. In Mobitz II the delay stays constant and a beat is dropped without any warning. Mobitz II carries a higher risk of progressing to complete heart block and usually calls for a pacemaker.
No. First-degree block, and Mobitz I block that causes no symptoms, are usually watched rather than treated. A pacemaker is the established treatment where the block causes symptoms such as fainting or severe fatigue, and for Mobitz II and complete heart block even when symptoms are mild. Before recommending a device, your cardiologist will look for a reversible cause, since medication, a thyroid problem or an electrolyte disturbance can each produce heart block that resolves once corrected.
Right bundle branch block is common and becomes more so with age, reaching around 11 per cent of people by the age of 80. On its own, in someone with no symptoms and no underlying heart disease, it is regarded as benign and needs no treatment. It is still worth a cardiology review the first time it appears, to confirm there is no heart condition behind it and to put a baseline tracing on file.
Left bundle branch block means the electrical signal travels down the left branch slowly or not at all, so the left side of the heart is activated late. It is much less common than right bundle branch block, affecting roughly 0.06 to 0.1 per cent of the general population, though it climbs steeply with age, reaching 6 to 7 per cent past 80. It is more often associated with an underlying heart condition, such as coronary artery disease, a weakened heart muscle or valve disease, so a newly found left bundle branch block warrants a proper look for a cause.
The commonest cause is age-related wear of the heart’s conduction system. Others include coronary artery disease and previous heart attack, medication that slows conduction such as beta-blockers and digoxin, cardiac surgery or a valve procedure, inflammation of the heart muscle, an underactive thyroid, and a raised potassium level. In fit young people, first-degree and Mobitz I block can appear during sleep as a normal effect of high vagal tone.
That depends on the grade, and on whether your heart rate rises normally with activity. Many people with first-degree block or an isolated bundle branch block exercise without any restriction. Where the block is higher grade, or where the rate fails to rise on exertion, your cardiologist will want to assess you before you take on hard exercise. If you have fainted, felt faint or become unusually breathless during activity, stop and have it assessed rather than working through it.
